 # Why the voltage of lithium-ion batteries cannot exceed 4.2V

We are writing this post about lithium-ion batteries 4.2V max voltage. You can find the other definition from Wikipedia.

The voltage of a lithium ion battery is determined by the electrode potential. Voltage, is a physical quantity that measures the energy difference of electric charges in an electrostatic field. The electrode potential of lithium ion is about 3V, and the voltage of lithium ion batteries varies with different materials. For example, a general lithium-ion battery has a rated voltage of 3.7V and a full-charge voltage of 4.2V; while a lithium iron phosphate battery has a rated voltage of 3.2V and a full-charge voltage of 3.65V. In other words, the potential difference between the positive electrode and the negative electrode of a lithium-ion battery in practical use cannot exceed 4.2V. Which is a requirement based on material and use safety.

If the Li/Li+ electrode is the reference potential, μA is the relative electrochemical potential of the negative electrode material. μC is the relative electrochemical potential of the positive electrode material. And the interval Eg is the lowest electron unoccupied energy level and the highest electron occupied energy of the electrolyte. The difference between the levels. Then, it is the three factors of μA, μC and Eg that determine the highest voltage value of a lithium-ion battery.

The difference between μA and μC is the open circuit voltage (the highest voltage value) of the lithium-ion battery. When this voltage value is in the Eg range, the electrolyte can be guaranteed to work normally. Normal operation means that the lithium-ion battery moves back and forth between the positive and negative electrodes through the electrolyte. But it does not undergo oxidation-reduction reactions with the electrolyte. Thereby ensuring the stability of the battery structure. The electrochemical causes the electrolyte to work abnormally in two ways:

1. When the electrochemical potential of the negative electrode is higher than the lowest electron unoccupied energy level of the electrolyte, the electrons of the negative electrode will be captured by the electrolyte. And the electrolyte will be oxidized, and the reaction product will form a solid-liquid interface layer on the surface of the negative electrode material particles. The negative electrode may be damaged.

2. When the electrochemical potential of the positive electrode is lower than the highest electron-occupied energy level of the electrolyte, the electrons in the electrolyte will be captured by the positive electrode and oxidized by the electrolyte. The reaction product forms a solid-liquid interface layer on the surface of the positive electrode material particles, resulting in The positive electrode may be damaged.

Now I understand that the open circuit voltage of lithium-ion batteries is chosen to be 4.2V because the current Eg range of commercial lithium-ion battery electrolyte is 1V~4.5V. If the open circuit voltage is set to 4.5V, the output of lithium-ion batteries may be improved. Electricity, but it also increases the risk of overcharging the battery, and the hazards of overcharging have been explained by a lot of data.

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